Method for the treatment of a flue gas stream

ABSTRACT

A sorbent composition such as for the removal of a contaminant species from a fluid stream, a method for manufacturing a sorbent composition and a method for the treatment of a flue gas stream to remove heavy metals such as mercury (Hg) therefrom. The sorbent composition includes a porous carbonaceous sorbent such as powdered activated carbon (PAC) and a solid particulate additive that functions as a flow-aid to enhance the pneumatic conveyance properties of the sorbent composition. The solid particulate additive may be a flake-like material, for example a phyllosilicate mineral or graphite.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit as a continuationapplication of U.S. patent application Ser. No. 15/295,665 filed Oct.17, 2016, now U.S. Pat. No. 10,137,403 which issued on Nov. 27, 2018,which is a divisional application of U.S. patent application Ser. No.14/145,731, filed on Dec. 31, 2013, now U.S. Pat. No. 9,468,904 whichissued on Oct. 18, 2016. All the contents of the aforementionedapplications are incorporated herein by reference in their entireties.

FIELD

This disclosure relates to the field of sorbent compositions, and inparticular to carbonaceous sorbent compositions such as powderedactivated carbon compositions that have improved pneumatic conveyancecapabilities.

BACKGROUND

Powdered activated carbon (“PAC”) is a highly amorphous form of carbonhaving an average particle size of about −80 mesh (e.g., not greaterthan about 177 μm). PAC may be manufactured to have a high porosity, asmall average pore size and a high surface area, and as such is able tobind (e.g., adsorb) many contaminant species from another substance,thereby purifying (e.g., decontaminating) the other substance.

Industrial applications of PAC include, but are not limited to, thetreatment of waste-water, food purification, and treatment of flue gasfrom a boiler (e.g., a coal boiler). In treatment of flue gas, PAC actsto remove contaminants such as mercury. For many such industrialapplications, PAC is shipped in bulk to an end user, such as by usingtrucks, rail shipment, or the like. The PAC is unloaded from the truckor rail car and may be placed subsequently in a storage unit (e.g., astorage silo). When the PAC is needed, it is then conveyed to the pointof use, e.g., to a water treatment process or to a flue gas treatmentprocess.

Often, the PAC is pneumatically conveyed, such as from a rail car to astorage unit, from a rail car to a transportation truck, or from atransportation truck to a storage unit, and/or from the storage unit tothe point of use. Pneumatic conveyance of PAC involves conveying the PACthrough an enclosed pipeline using a pressure differential and the flowof a gas (e.g., air) to suspend and move the PAC along the pipeline.Typically, the PAC is conveyed in a dilute phase, i.e., where theconveying system relies on the gas velocity to pick up and entrain theparticles. Pneumatic conveyance of PAC has many advantages over otherconveyance techniques. For example, pneumatic conveyance pipelines canbe arranged with bends to circumvent other equipment, and the system hasfew moving parts and is completely enclosed.

SUMMARY

Recently, it has been found that for some applications, and for thetreatment of flue gas streams in particular, particulate sorbentcompositions (e.g., PAC-containing sorbent compositions) having areduced average particle size may be advantageous for the capture ofcontaminants such as mercury (Hg) from the flue gas stream. Inparticular, PAC compositions having a relatively small average particlesize, such as a median particle size (D50) of about 15 μm or less, maybe advantageous for the efficient removal of mercury from a flue gasstream.

However, it has been found that some PAC-containing sorbent compositionshaving a relatively small average particle size may be difficult topneumatically convey in some situations, such as from a storage unit tothe point of use (e.g., to the lances that inject the PAC-containingsorbent composition into the flue gas stream). Specifically, it has beenfound that such compositions may experience flow interruption issuesduring pneumatic conveyance, which can result in feeder and/or blowershut down for a period of time due to pressure spikes in the conveyancelines and/or other flow irregularities. Such shut downs may result in aninterruption of the PAC flow to the flue gas stream and may lead tomercury emission problems. A need has been identified for a sorbentcomposition containing PAC having a relatively small median (D50)particle size and with good pneumatic conveyance properties, i.e., toreduce interruptions during pneumatic conveyance of the sorbentcomposition.

Disclosed herein are sorbent compositions that include a relativelysmall particle size PAC as a sorbent material, where the composition isformulated to enhance the pneumatic conveyance properties of thecomposition, particularly when pneumatically conveyed in a dilute phase,without any significant loss of adsorptive performance. In oneembodiment, a sorbent composition is provided that includes aparticulate blend of at least about 75 wt. % of a porous carbonaceoussorbent having a median particle size (D50) of at least about 4 μm andnot greater than about 15 μm. The sorbent composition also includes atleast about 0.1 wt. % and not greater than about 5 wt. % of a solidparticulate additive, where the solid particulate additive has at leastone particle property selected from: (i) a median aspect ratio of notgreater than 0.7; (ii) a median circularity of not greater than 0.92; or(iii) a median elongation factor of at least 0.3.

In one characterization, the sorbent composition comprises powderedactivated carbon, and may consist essentially of powdered activatedcarbon. In another characterization, solid particulate additive has amedian aspect ratio of not greater than 0.7, such as not greater than0.67. In another characterization, the solid particulate additive has amedian circularity of not greater than 0.92, such as not greater than0.90. In yet another characterization, the solid particulate additivehas a median elongation factor of at least 0.3, such as at least 0.33.

In another characterization, the sorbent composition has a Basic FlowEnergy (BFE) of not greater than about 330 mJ, as measured by a powderrheometer using the Stability and Variable Flow Rate test described indetail below. In another characterization, the sorbent composition has aBFE of not greater than about 265 mJ.

In another characterization, the sorbent composition has a SpecificEnergy (SE) of not greater than 7.5 mJ/g, as measured by a powderrheometer using the Stability and Variable Flow Rate test. In anothercharacterization, the SE of the sorbent composition is not greater than7 mJ/g.

The sorbent compositions may also be characterized as having an AerationEnergy AE) of not greater than 30 mJ, during the aeration test, asmeasured by a powder rheometer, such as by having an AE of not greaterthan 24 mJ or even an AE of not greater than about 10 mJ.

In another characterization, porous carbonaceous sorbent has a medianparticle size of not greater than about 12 μm, such as from about 8 μmto about 12 μm. The median particle size of the solid particulateadditive may be at least about 1 μm. For example, the solid particulateadditive may be characterized as having a median particle size that isgreater than or is equal to the median particle size of the porouscarbonaceous sorbent. In another characterization, the solid particulateadditive may have a median particle size that is less than the medianparticle size of the sorbent.

In one characterization, the solid particulate additive may be selectedfrom the group consisting of phyllosilicate minerals, graphite, andmixtures thereof. In one particular characterization, the solidparticulate additive may be selected from the group consisting of mica,talc, graphite, and mixtures thereof, and in another particularcharacterization the solid particulate additive is graphite. The sorbentcomposition may be characterized as comprising not greater than about 5wt. % of the solid particulate additive, such as comprising not greaterthan about 3 wt. % of the solid particulate additive or even comprisingnot greater than about 2 wt. % of the solid particulate additive.

In another characterization, the addition of the solid particulateadditive to the sorbent composition reduces fluctuation in line pressureduring pneumatic conveyance as measured by standard deviation of lessthan ±0.050 psi. In another characterization, the solid particulateadditive reduces fluctuation in line pressure of the sorbent compositionduring pneumatic conveyance as measured by a difference between maximumto minimum line pressure of less than 0.1 psi, such as a differencebetween maximum to minimum line pressure of less than 0.08 psi, or evenless than 0.06 psi.

In another embodiment, a sorbent composition is provided that comprisesa particulate blend of at least about 50 wt. % powdered activated carbonsorbent having a median particle size (D50) of at least about 5 μm andnot greater than about 15 μm, and at least about 0.1 wt. % and notgreater than about 2 wt. % of a solid particulate additive selected fromthe group consisting of phyllosilicate minerals, graphite and mixturesthereof.

In one characterization, the solid particulate additive is selected fromthe group consisting of mica, talc, graphite, and mixtures thereof, andin one particular characterization the solid particulate additivecomprises graphite. The graphite may have a median particle size of atleast about 3 μm and not greater than about 5 μm, for example.

According to another embodiment, a method for the treatment of a fluegas stream to remove heavy metals therefrom is provided. The methodincludes the steps of pneumatically conveying a sorbent composition froma storage unit to a point of injection; and injecting the sorbentcomposition into a flue gas stream. The sorbent composition comprisespowered activated carbon and an effective amount of a solid particulateadditive such that the sorbent composition has a Basic Flow Energy ofnot greater than about 350 mJ.

In one characterization, the sorbent composition has a Specific Energyof not greater than about 7.5 mJ/g. In another characterization, thesorbent composition has an Aeration Energy of not greater than about 35mJ. In yet another characterization, the sorbent composition has a basicflow energy of not greater than about 300 mJ.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a system and method forpneumatically transporting and introducing a sorbent composition into aflue gas stream at an electric generating unit (EGU) or industrialboiler site.

FIG. 2 illustrates the standard deviation of pressure readings duringpneumatic transportation of sorbent compositions in a pilot plantsystem.

FIG. 3 illustrates difference between the maximum pressure reading tothe minimum pressure reading during pneumatic transportation of sorbentcompositions in a pilot plant system.

FIGS. 4A and 4B illustrate scanning electron microscope (SEM) images ofgraphite.

FIGS. 5A and 5B illustrate SEM images of mica.

FIGS. 6A and 6B illustrate SEM images of talc.

FIGS. 7A and 7B illustrate SEM images of silica.

FIG. 8 illustrates an aspect ratio distribution for several differentsolid particulate additives.

FIG. 9 illustrates an elongation distribution for several differentsolid particulate additives.

FIG. 10 illustrates a circularity distribution for several differentsolid particulate additives.

DESCRIPTION OF THE INVENTION

Disclosed herein are sorbent compositions, methods of making a sorbentcompositions, and methods for using sorbent compositions. The sorbentcompositions may include a porous carbonaceous material such as PAC as asorbent material, and the sorbent compositions may be particularlyformulated to remove contaminants from a fluid stream, such as to removeheavy metals, e.g., mercury (Hg), from a flue gas emanating from aboiler (e.g., from a pulverized coal boiler or a waste incinerator). Inaddition to PAC, the sorbent compositions may include a solidparticulate additive that is selected to decrease the energy required topneumatically convey the sorbent composition, e.g., from a storage unitto the point of injection. Although the sorbent compositions disclosedherein are described with particular reference to flue gas treatment,the sorbent compositions may be used in other applications, includingbut not limited to waste water treatment and other industrialpurification processes.

When a combustible material containing fixed carbon and volatile matter(e.g., coal or municipal waste) is combusted in a boiler (e.g., for thegeneration of electricity using a steam turbine), a flue gas is formedthat includes contaminants that originate from the combustible material.Such contaminants may include mercury, e.g., in concentrations of fromabout 1 ppbw to 100 ppbw (parts per billion by weight). Environmentalregulations limit the concentration of mercury in the flue gas that canbe vented to the atmosphere. One method to capture mercury from the fluegas is to contact the flue gas with a sorbent such as PAC, e.g., byinjecting PAC into the flue gas stream. PAC is known to effectivelyremove heavy metals such as mercury from such fluid streams.

FIG. 1 schematically illustrates a system 100 and method forpneumatically transporting and introducing a particulate sorbentcomposition into a flue gas stream 102 at a power plant or industrialboiler site, such as a coal-burning facility. A boiler 101 is configuredto burn coal and/or another material (e.g., municipal waste). Theburning of coal in the boiler 101 creates a flue gas stream 102 that mayinclude various components including gases (e.g., N₂, CO₂, O₂, NO_(x),SO_(x) etc.) and particulates (e.g., fly ash). The flue gas typicallyalso includes heavy metals such as Hg that must be removed from the fluegas stream 102 before exiting the stack 104. In this regard, sorbentcompositions such as PAC may be injected into the flue gas stream 102 tocapture (e.g., sequester) mercury.

PAC may be stored in a silo or hopper 105 prior to introduction to theflue gas stream 102. A blower 106 forces air through an eductor 107creating a venturi effect dispersing the PAC into a conveyance line 108and through injection lances 109 into the flue gas stream 102. Further,a screw feeder (not shown) may transport PAC from the silo or hopper 105to the eductor 107. After contacting the flue gas stream 102 with thePAC, the flue gas stream may be introduced to a particulate matterseparation unit 103 to remove particulates (e.g., PAC and fly ash)before exiting the stack 104. This particulate matter separation unit103 may be, for example, an electrostatic precipitator. It will beappreciated by those skilled in the art that the plant may include otherdevices not illustrated in FIG. 1, such as a selective catalyticreduction unit (SCR), an air preheater, and the like, and may havenumerous other configurations.

PAC having a median particle size of about 30 μm or greater generallyperforms well in such a system, i.e., the PAC does not typically presentpneumatic conveyance problems when being pneumatically conveyed throughconveyance line 108. However, use of smaller median particle size PAC,which may provide enhanced mercury capture performance from flue gasstreams, may also result in interruptions in delivery of the PAC to theflue gas stream (e.g., due to plugging).

To address these issues, the sorbent composition disclosed herein mayinclude a particulate blend of at least two components; a porouscarbonaceous sorbent such as PAC and a solid particulate additive thatis selected to improve the ability to pneumatically convey the sorbentcomposition. Although described herein with particular reference to PAC,it is to be understood that the sorbent may include other porouscarbonaceous materials, i.e., porous sorbents that include a substantialamount of fixed carbon, such as reactivated carbon or carbonaceous char.

To enhance the efficacy of the flue gas treatment to remove heavymetals, the PAC may be characterized as having a relatively smallaverage particle size, e.g., a relatively small volume average medianparticle size (D50). In this regard, the PAC may have a median particlesize of not greater than about 20 μm, such as not greater than about 15μm, or even not greater than about 12 μm. However, for material handlingpurposes, the PAC particles should not be too small, and in anothercharacterization the PAC may have a median particle size of at leastabout 2 μm, such as at least about 4 μm, such as at least about 6 μm, oreven at least about 8 μm. One exemplary median particle size range maybe from about 4 μm to about 15 μm, which may be further exemplified by amedian particle size range of from about 8 μm to about 12 μm. Arelatively small median particle size, such as not greater than about 15μm, typically means greater surface area per volume of the PAC. Theincreased surface area (e.g., as compared to PACs with a larger medianparticle size) may provide many benefits, including, but not limited to,increased surface area available for reactions to occur, and thusoverall improved diffusion and reaction kinetics and mercury captureperformance. The median particle size may be measured using techniquessuch as light scattering (e.g., using a Saturn DigiSizer II, availablefrom Micromeritics Instrument Corporation, Norcross, Ga.).

In addition to the relatively small median particle size, the PAC mayalso have a relatively high pore volume and a well-controlled sizedistribution of the pores, particularly among the mesopores (i.e., from20 Å to 500 Å diameter) and the micropores (i.e., not greater than 20 Ådiameter). A well-controlled distribution of micropores and mesopores inthe PAC is desirable for effective removal of contaminants from a fluidstream, such as for mercury removal from a flue gas stream. In onecharacterization, the sum of micropore volume plus mesopore volume ofthe PAC may be at least about 0.10 cc/g, such as at least 0.20 cc/g, andat least about 0.25 cc/g or even at least about 0.30 cc/g. In anothercharacterization, the micropore volume of the PAC may be at least about0.10 cc/g, such as at least about 0.15 cc/g. Further, the mesoporevolume of the PAC may be at least about 0.10 cc/g, such as at leastabout 0.15 cc/g. In a further characterization, the ratio of microporevolume to mesopore volume may be at least about 0.7, such as 0.9, andmay be not greater than about 1.5. Such levels of micropore volumerelative to mesopore volume advantageously enable efficient capture andsequestration of oxidized mercury species, such as HgBr₂, by the PAC.Pore volumes may be measured by gas adsorption techniques (e.g., N₂adsorption) using instruments such as a TriStar II Surface Area Analyzer(Micromeritics Instruments Corporation, Norcross, Ga., USA).

The PAC may also be characterized as having a well-controlled particledensity. Particle density is the mass of the particle over the volume ofthe particle (excluding pore volume of larger pores that do notcontribute to adsorption), and is typically measured in g/cc. Particledensity correlates to the surface area to volume ratio of the PAC, whichin turn affects mercury capture performance. Particle density can bemeasured by liquid mercury volume displacement, in which case the resultis referred to as the mercury particle density. In this regard, the PACmay have a mercury particle density of at least about 0.5 g/cc, such asat least about 0.6 g/cc. Conversely, the mercury particle density of thePAC may be not greater than about 0.9 g/cc, such as not greater thanabout 0.8 g/cc. Mercury particle density may be measured using aMicromeritics AccuPyc Pycnometer (Micromeritics Inc., Norcross, Ga.,USA).

Particle density may also be measured by sedimentary volumedisplacement, in which case the result is referred to as the envelopeparticle density. In this regard, the envelope particle density of thePAC may be at least about 0.5 g/cc, such as at least about 0.6 g/cc orat least about 0.7 g/cc. The envelope particle density of the PAC may benot greater than about 1.0 g/cc, such as not greater than about 0.9g/cc. Envelope particle density may be measured using a MicromeriticsGeoPyc Envelope Density Analyzer (Micromeritics, Inc., Norcross, Ga.,USA).

The foregoing description of the PAC pore characteristics and densitycharacteristics are being presented as exemplary embodiments, and itwill be appreciated that the present disclosure is not limited to theuse of PAC or other carbonaceous sorbents having these specificcharacteristics.

As is discussed above, PACs having a small median particle size (e.g.,not greater than about 15 μm) may be difficult to pneumatically conveyin some situations. Flow interruptions and irregularities may lead tofeeder and blower shut down in the sorbent conveyance system, such asdue to pressure spikes. Such interruptions may be due to cohesiveness ofthe powder, mechanical interlocking of the particles, static chargebuildup, or other factors. Such instabilities and interruptions can beparticularly problematic when the PAC is being conveyed from a storageunit to the injection lances that inject the PAC into the flue gasstream, as flow interruptions can cause the flue gas to pass through thestack without contacting a sufficient amount of PAC to remove mercuryfrom the flue gas. In this regard, the sorbent compositions disclosedherein may also include a solid particulate additive, e.g., that isblended with (e.g., dispersed throughout) the PAC, where the solidparticulate additive is selected to enhance the ability to pneumaticallyconvey the sorbent composition, e.g., to reduce the possibility of flowinterruptions.

Silica has been used as a flow additive, particularly at small medianparticle sizes, e.g., much less than 1 μm, to improve the flowproperties of some powders. These fine-sized silica particles tend toagglomerate to form larger and generally spherical (e.g.,non-flake-like) agglomerates. However, it is has been found that theability to pneumatically convey a sorbent composition containing PAC maybe enhanced when the solid additive particles are non-spherical (e.g.,anisometric) in shape, and in particular when the solid additiveparticles are predominately flake-like.

In one aspect, the solid particulate additive may be selected from thosematerials that form such sheet or flake-like morphologies naturally dueto their inherent structure, e.g., their inherent morphology. The solidparticulate additive may be predominately crystalline (e.g., greaterthan 50% crystalline), as non-crystalline (i.e., amorphous) materials donot form such flake-like morphologies naturally. The solid particulateadditive may also be characterized as having well defined cleavageplanes. Examples of such materials include phyllosilicates (i.e., sheetsilicates). Phyllosilicates are a group of minerals that includes themicas, chlorite, serpentine, talc, and the clay minerals. Among these,clay minerals (e.g., calcined clay minerals), aluminosilicates (e.g.,calcined aluminosilicates), mica group minerals and talc (hydratedmagnesium silicate) may be particularly useful as the solid particulateadditive to enhance the pneumatic flow properties of the sorbentcomposition. Another example of a solid particulate additive isgraphite. Among these, mica, talc and graphite may be particularlyuseful. Graphite also has an advantage in that it has favorableelectrical properties that may make the sorbent composition easier toremove from the flue gas using particulate control devices such as anESP. In one particular embodiment, the solid additive particles includenatural graphite, e.g., natural flake graphite. The natural graphite mayoptionally be beneficiated (e.g., milled) to achieve a desired medianparticle size.

The sorbent compositions disclosed herein may include various amounts ofthe porous carbonaceous sorbent (e.g., PAC) and the solid particulateadditive, provided that there is a sufficient amount of the PAC toeffectively remove contaminants from a fluid stream (e.g., to removemercury from a flue gas stream) and a sufficient amount of the solidparticulate additive to enhance the pneumatic flow properties of thePAC. In one embodiment, the sorbent composition includes at least about50 wt. % of the porous carbonaceous sorbent, such as at least about 60wt. %, at least about 75 wt. %, at least about 80 wt. % or even at leastabout 85 wt. %. However, the sorbent compositions will typically includenot greater than about 99 wt. % of the porous carbonaceous sorbent, suchas not greater than about 98 wt. %.

It is an advantage that the pneumatic conveyance properties of thesorbent composition can be improved even with the addition of arelatively small amount of the solid particulate additive. Becauserelatively small amounts of the solid particulate additive are required,the ability of the sorbent composition to remove (e.g., adsorb)contaminant species from a fluid stream is not substantially diminishedby the presence of the additive. In this regard, the sorbent compositionmay include not greater than about 5 wt. % of the solid particulateadditive, such a not greater than about 3 wt. %, or even not greaterthan about 2 wt. %. However, it may be necessary to include at leastabout 0.1 wt. %, such as at least about 0.5 wt. %, of the solidparticulate additive in the sorbent composition to provide a sufficientimprovement in the pneumatic conveyance properties.

Scanning electron microscopy (SEM) may be used to characterize the shapeof the particulate additives. A particle's shape can also be quantifiedusing various shape factors, which are dimensionless quantities used inimage analysis and microscopy. The dimensionless quantities oftenrepresent the degree of deviation from an ideal shape, such as a circle,sphere, or equilateral polyhedron. Shape factors are often normalized,that is, the value ranges from zero to one. A shape factor equal to oneusually represents an ideal case or maximum symmetry, such as a circle,sphere, square or cube. Shape factors may be calculated from measureddimensions, such as diameter, chord length, area, perimeter, centroid,moments, and the like. The dimensions of the particles are usuallymeasured from two-dimensional cross-sections or projections, as in amicroscope field, but shape factors also apply to three-dimensionalobjects. Shape factors are independent of the particle size.

A few shape factors that may be pertinent in distinguishingnon-flake-like particles (e.g., spherical-like) such as silica from theflake-like particulate additives (e.g., phyllosilicates and graphite)disclosed herein are aspect ratio, elongation, and circularity. Theseshape factors may be determined based on particle length, area, widthand perimeter measurements obtained using automated imaginginstrumentation (e.g., the Malvern MorphologiG3, Malvern InstrumentsLimited, Worcestershire, UK). Such instruments may be utilized to firstdetermine the major axis and minor axis of the particles. The major axispasses through the center of mass of the imaged object at an orientationcorresponding to the minimum rotational energy of the shape. The minoraxis passes through the center of mass at a right angle to the majoraxis. Length is the longest line between two points on the perimeterthat is also parallel to the major axis. Width is the longest linebetween two points on the perimeter that is also parallel to the minoraxis. The perimeter is the total length of the object boundarycalculated by summing the length of the boundary pixels. This includesan adjustment to take account of direction changes. The area is thevisual projected area of the particle. It should be noted, however, dueto the technical challenges of trying to determine three-dimensionalshape characteristics from a two dimensional image, and due to the factthat sheet or flake-like objects will tend to lie in a manner difficultto determine its depth, no single factor is dispositive and it is bestto consider all of the relevant factors in totality.

Aspect ratio is the ratio of the width to the length of the particlecalculated as:

${{Aspect}\mspace{14mu}{Ratio}} = \frac{width}{length}$Particles (or particle agglomerates) that have a spherical shape (i.e.,lengths are similar to widths) will tend to have aspect ratios closer toone because the aspect ratio of a sphere (e.g., the two-dimensionalrendition of a sphere) is one. Also, analysis of a large distribution ofgenerally spherical-like particles will tend to give aspect ratios closeto one because the random orientations will tend to average out. Incomparison, phyllosilicates and graphite (e.g., flake-like particles)with organized structures tend to demonstrate an aspect ratio of lessthan one. As such, the median aspect ratio of the solid particulateadditive disclosed herein may be not greater than about 0.7, such as notgreater than about 0.68, such as not greater than about 0.66, such asnot greater than about 0.63.

The elongation is yet another way to describe particle shape. Elongationis calculated as 1 minus the aspect ratio:

${Elongation} = {1 - \frac{width}{length}}$A particle with a greater difference between its width and length willtend to have an elongation factor further from zero. As such, the medianelongation of the solid particulate additive may be at least about 0.30,such as at least about 0.33, or even at least about 0.35, such as atleast about 0.37.

Circularity is measured by the ratio of the area of the particle dividedby the area of a circle that has the same perimeter as the particle. Itcan be calculated as:

${Circularity} = \frac{2\sqrt{\Pi \times {Area}}}{Perimeter}$Particles that have a spherical shape will tend to have circularitycloser to one. Also, an analysis of a large distribution of amorphousparticles will also tend to give a circularity close to one because therandom orientations will tend to average out. In comparison,phyllosilicates and graphite (e.g., flake-particles) with organizedstructures tend to demonstrate a circularity of less than or furtherfrom one. Thus, the median circularity of the solid particulate additivedisclosed herein may be not greater than about 0.92, such as not greaterthan about 0.90, not greater than about 0.88, not greater than about0.86, or even not greater than about 0.84.

Thus, the solid particulate additives disclosed herein may becharacterized as having any one of the foregoing shape factors, or anycombination of two or three of the foregoing shape factors. Thus, in oneembodiment, the solid particulate additive may be characterized ashaving at least one particle property (e.g., shape factor) selected froma median aspect ratio of not greater than 0.7, a median circularity ofnot greater than 0.92, and a median elongation factor of at least 0.3.

It is also advantageous that the pneumatic conveyance properties of thesorbent composition may be improved with even small amounts of the solidparticulate additive in the sorbent composition. In this regard, thesolid particulate additive can have little or no effect on the abilityof the sorbent composition (e.g., the PAC) to capture mercury from aflue gas stream. In one aspect, the sorbent composition may comprise notgreater than about 2.0 wt. % of the solid particulate additive, such asnot greater than about 1.5 wt. % or even not greater than about 1.0 wt.%. In another aspect, the sorbent composition may comprise as little asat least about 0.1 wt. % of the solid particulate additive, such as atleast about 0.25 wt. %, or at least about 0.5 wt. %.

The difference between median particle size of the solid particulateadditive and the median particle size of the sorbent is not believed tobe of significant consequence to enhancing the pneumatic conveyancecharacteristics of the sorbent composition. In one characterization, themedian particle size of the solid particulate additive is smaller thanthe median particle size of the PAC. For example, in onecharacterization, the solid particulate additive may have a medianparticle size that is smaller than the median particle size of the PAC,such as where the particulate additive includes graphite having a medianparticle size of from about 3 μm to about 5 μm. In anothercharacterization, the median particle size of the solid particulateadditive, for example talc, is from about 12 μm to about 27 μm, and maybe equal to or greater than the median particle size of the PAC. Inanother characterization, the solid particulate additive includes micahaving a median particle size of from about 10 μm to about 17 μm, wherethe median particle size is about the same or slightly less than or morethan the median particle size of the PAC.

In addition to the PAC and the solid particulate additive, the sorbentcompositions may include other additives that are known to those skilledin the art, particularly those used to enhance the sequestration ofmercury from a flue gas stream. For example, the sorbent compositionsmay also include an oxidizing agent that is adapted to oxidize theelemental mercury in the flue gas stream to an oxidized mercury species(e.g., Hg²⁺) that may be captured and sequestered by the PAC. Theoxidizing agent may be a halogen oxidizing agent, for example. In thisregard, the oxidizing agent may include a halogen that is selected fromthe group consisting of fluorine, chlorine, bromine, iodine or acombination thereof. For example, the halogen oxidizing agent may bepresent in the sorbent composition as a halogen salt, such as a bromidesalt (e.g., sodium bromide) or other bromine compound or moiety. Brominemay react with the elemental mercury to form an oxidized species such asHgBr₂. For example, the sorbent composition may include at least aboutone weight percent, such as at least 2 weight percent of a halogenoxidizing agent.

Other additives to the sorbent composition may include those additivesthat are selected to increase the tolerance of the sorbent compositionto use in flue gas streams containing acid gases (e.g., containingH₂SO₄) and/or acid-gas precursors (e.g., SO_(x)). Such acid gasadditives may include a metal cation selected from the group consistingof Group 3 to Group 14 metals. In one characterization, the metal cationis selected from the Group 13 and Group 14 metals. For example, themetal cation may be a Group 13 metal, and in a particularcharacterization may be aluminum. In another particularcharacterization, the metal cation may be tin. The salt may be aninorganic salt, and may include an anion selected from the groupconsisting of hydroxides, oxides, and carbonates. In one particularcharacterization, the salt may comprise aluminum hydroxide (Al(OH)₃). Inanother characterization, the salt may be selected from the groupconsisting of tin oxide (SnO₂) and tin hydroxide (Sn(IV)(OH)₄) the saltmay have an anion or polyatomic anion that comprises an atom having avalency of 3 or higher, such as a valency of +3 or +4. The anion maycomprise a metal. For example, the metal may be selected from the groupconsisting of Group 3 to Group 14 metals. In one characterization, themetal may be selected from the group consisting of Group 3 to Group 12metals, or may be selected from the group consisting of Group 13 andGroup 14 metals. For example, the metal may be a Group 13 metal, and ina particular characterization may be aluminum. In another particularcharacterization, the metal may be tin. In one particularcharacterization, the salt may comprise sodium aluminate (e.g., NaAlO₂or Na₂O.Al₂O₃), including hydrated sodium aluminate (e.g., NaAl(OH)₄).In another characterization, the salt may comprise sodium stannate(Na₂Sn(OH)₆) calcium oxide (CaO), quicklime, also known as hydratedlime, used in flue gas to control SO₃, and neutralize acid gases.

As is noted above, the sorbent compositions disclosed herein (e.g.,including a solid particulate additive) may advantageously have improvedpneumatic conveyance properties as compared to known sorbentcompositions, particularly when the sorbent (e.g., the PAC) has arelatively small median particle size (e.g., 15 μm or less). Among thebulk powder characteristics that can be quantitatively measured toindicate such an improvement are the Basic Flow Energy (“BFE”), theSpecific Energy (“SE”), and the Aeration Energy (“AE”) of the sorbentcomposition.

These bulk powder characteristics can be measured using a rheometer, forexample using a Freeman Technology 4 (“FT4”) Powder Rheometer, availablefrom Freeman Technology (Worcestershire, United Kingdom). The FT4 PowderRheometer is capable of quantitatively measuring the flowabilitycharacteristics of sorbent compositions, and these measurements can beutilized to predict the characteristics of the sorbent composition whenbeing pneumatically conveyed, e.g., in a dilute phase. The FT4 PowderRheometer includes a container for holding a powder sample and a rotorhaving a plurality of blades that is configured to move in the axialdirection (e.g., vertically) through the powder sample while rotatingrelative to the container. See, for example, U.S. Pat. No. 6,065,330 byFreeman et al., which is incorporated herein by reference in itsentirety.

BFE and SE may be determined using a FT4 Powder Rheometer via the commonStability and Variable Flow Rate method (“the SVFR method”). The SVFRmethod includes seven test cycles using a stability method and four testcycles using a variable flow rate method, where each test cycle includesa conditioning step before the measurement is taken. The conditioningstep homogenizes the sorbent composition by creating a uniform lowstress packing throughout the sample, which removes any stress historyor excess entrained air prior to the measurement. The stability methodincludes maintaining the blade tip speed at about 100 millimeters persecond (mm/s) during the measurement, whereas the variable flow ratemethod involves four measurements using different blade tip speeds,namely about 100 mm/s, about 70 mm/s, about 40 mm/s and about 10 mm/s.The test measures the energy required to rotate the blade through thepowder from the top of the vessel to the bottom and to rotate the bladethrough the powder from the bottom to the top of the vessel.

The BFE is a measurement of the energy required to displace a precisevolume of (conditioned) powder at a given flow pattern, flow rate, andblade tip speed. Stated another way, the BFE may be considered ameasurement of the “flowability” of the sorbent composition. For manytypes of dry powders, including dry powders that incorporate aparticulate additive such as a flow aid, the lower the BFE, the moreeasily the sorbent composition can be made to flow. BFE is the totalenergy required to create a defined flow pattern, measured during theseventh cycle during the stability method measurements of the SVR methoddescribed above, at a tip speed set at −100 mm/s, while the blade isrotating from the top of the vessel to the bottom.

Thus, it has been found that the sorbent compositions disclosed hereinhave improved pneumatic conveyance properties, such as when compared tothe same or similar compositions that do not incorporate the particulateadditive. One way that the present sorbent compositions may becharacterized is that the sorbent compositions may have a BFE of notgreater than about 350 millijoules (mJ), such as not greater than about330 mJ, not greater than about 300 mJ, not greater than about 275 mJ, oreven not greater than about 265 mJ.

The SE is the energy required to establish a particular flow pattern ina conditioned, precise volume of powder in an unconfined stressenvironment. In contrast to the BFE, the flow pattern is generated by anupward, clockwise motion of the blade in the rheometer, generatinggentle lifting and low stress flow of the powder. SE is the total energymeasured during the seventh cycle during the stability methodmeasurements of the SVR method described above, at a tip speed set at−100 mm/s, while the blade is rotating from the bottom of the vessel tothe top. As with the BFE, for many types of dry powders, including drypowders that incorporate a particulate additive such as a flow aid, thelower the SE, the more easily the sorbent composition can be made toflow. Thus, a lower SE may indicate better flowability, particularly inlow stress environments. In this regard, the sorbent compositionsdisclosed herein may have a SE of not greater than about 7.5 millijoulesper gram (mJ/g), such as not greater than about 7 mJ/g.

AE is how much energy is required for a powder to become aerated, whichis directly related to the cohesive strength of the powder (i.e., thetendency for particles to stick together). It may be determined in theFT4 Powder Rheometer using the aeration test, which provides a series ofprecise air velocities to the base of the vessel containing the powder,and measures the reduction in flow energy at each velocity. During theaeration test, the air velocity (e.g., in mm/s) is varied over a rangeof about 0.2 mm/s to about 2.0 mm/s, in 0.2 mm/s increments. AE is theflow energy at 2.0 mm/s.

Thus, a lower AE generally indicates that a powder may be more easilypneumatically conveyed. In this regard, the sorbent compositionsdisclosed herein may have an AE of not greater than about 35 mJ, such asnot greater than about 30 mJ, not greater than about 24 mJ, or even notgreater than about 10 mJ.

In one example, the compositions may be characterized as having areduced fluctuation in line pressure during pneumatic conveyance of thecompositions as compared to the same or similar composition without theaddition of a solid particulate additive disclosed herein. Thefluctuation in line pressure may be described in terms of the standarddeviation of the line pressure during pneumatic conveyance. In onecharacterization, the standard deviation in line pressure when using thesorbent compositions disclosed herein is not greater than about 0.050pounds per square inch (psi), such as not greater than about 0.045 psi.Characterized another way, the standard deviation in line pressure isreduced by at least about 25%, such as by at least about 35%, ascompared to the same or similar sorbent composition without the additionof a solid particulate additive described herein.

In another example, the fluctuation or instability in line pressure maybe described in terms of the difference (delta) between the minimum andmaximum line pressure during pneumatic conveyance of the composition. Inone characterization, the difference between the minimum and maximumline pressure using the sorbent compositions disclosed herein is notgreater than about 0.1 psi, such as not greater than about 0.08 psi, oreven not greater than about 0.06 psi.

EXAMPLES

Samples of sorbent compositions are prepared to evaluate the pneumatictransport capability of the sorbent compositions disclosed herein ascompared to known sorbents. Comparative Sample 1 is a prior art sorbentcomposition that includes PAC and has a median particle size of about 30μm and performs well at EGU sites with respect to pneumatic conveyance.Comparative Sample 2, is a prior art sorbent composition that includesPAC and has a median particle size below 15 μm and shows improvedmercury capture capability. However, due to the small particle size, thesorbent may be susceptible to pneumatic conveyance interruptions. Sample3 is a sorbent composition made by the addition of 1.0 wt. % graphite tocomparative Sample 2 in bulk volume. Comparative Sample 4 is acomparative sorbent composition that includes PAC and has a medianparticle size of about 30 μm and performs well at EGU sites with respectto pneumatic conveyance. Sample 5 is a sorbent composition according tothe present disclosure wherein 0.5 wt. % graphite is mixed withcomparative Sample 2 at lab scale. Sample 6 is also an example sorbentcomposition according to the present disclosure wherein 1.0 wt. %graphite is mixed with comparative Sample 2 at lab scale. Sample 7 isalso a sorbent composition according to the present disclosure wherein0.5 wt. % mica is mixed with comparative Sample 2 at lab scale. Sample 8is a sorbent composition according to the present disclosure, wherein0.5 wt. % talc is mixed with comparative Sample 2 at lab scale.

Example 1

Example 1 illustrates the ability of a sample of the sorbentcompositions disclosed herein to exhibit improved flow properties inpneumatic conveyance systems as compared to comparative Sample 1, suchas at an EGU or industrial boiler site. A demonstration system is builtto simulate the pneumatic conveyance of the sorbent composition in anindustrial system such as that illustrated in part in FIG. 1, i.e., fromthe hopper 105 through the conveyance line 108. Specifically, thedemonstration system includes a first receiving hopper that receivessorbent composition from a bulk bag containing at least 750 lbs. ofproduct. A blower and an eductor are used to convey the sorbentcomposition into a flexible hose. The hose, at about 400′ in length,transports the sorbent composition to a trailer. The pressure (psi) ismeasured approximately 3′ after the eductor. The 400′ hose is laid outto include seven hard bends and four soft bends. Hard bends are definedas those making a 90 degree turn or less. Soft bends are defined asthose making greater than a 90 degree turn. Tests are run by injecting asample at a specified loading rate (e.g., 300, 500 and 700 pounds perhour (PPH), and line pressure (psi) is measured once a minute for 30minutes.

FIG. 2 illustrates the standard deviation of line pressure (psi) foreach of comparative Samples 1 and 2, with Sample 3. The standarddeviation of Sample 3 which included 1.0 wt. % graphite additive moreclosely matches that of the comparative Sample 1, a sorbent material oflarger median particle size with no known pneumatic conveyance issues.Comparative Sample 2, with essentially the same particle size PAC asSample 3, exhibits a much higher standard deviation in line pressure.

FIG. 3 illustrates the difference (delta) between maximum and minimumline pressures in pounds per square inch (psi) for each of comparativeSamples 1 and 2, with Sample 3. The difference of maximum to minimumline pressures of Sample 3 more closely matched that of the comparativeSample 1 a sorbent material of larger median particle size with no knownpneumatic conveyance issues. The comparative Sample 2, with essentiallythe same particle size PAC as Sample 3, exhibits a much higherdifference in line pressure than Sample 3.

Example 2

Example 2 illustrates particle shape data for the flake-like solidparticulate additives disclosed herein as compared to silica, whichincludes very small (e.g., sub-micron) particles that have a propensityto agglomerate and thus form circular/spherical agglomerates. A JEOLJSM-7000F field emission scanning electron microscope (FESEM) is used tovisualize the particle morphology of the additives disclosed herein,particularly as compared to the particle morphology of materials such assilica. A 5 kV accelerating voltage was used to visualize the graphite,mica, and talc samples. A gold coated sample of hydrophobic silica(SiO₂) was visualized with 2 kV accelerating voltage. FIGS. 4-7illustrate the morphology of flake-like particulate additives inaccordance with the present disclosure. Specifically, FIGS. 4A and 4Billustrate the particle morphology of graphite at magnifications of2000× and 4000×, respectively. FIGS. 5A and 5B illustrate the particlemorphology of mica at magnifications of 2000× and 4000×, respectively.FIGS. 6A and 6B illustrate the particle morphology of talc atmagnifications of 2000× and 4000×, respectively. In each of these FESEMimages, it can be seen that the particles exhibit a flake-likemorphology, and tend to be at least several microns in size. Incomparison, FIGS. 7A and 7B illustrate the morphology of silicaparticles (Aerosil® R 974, a hydrophobic fumed silica available fromEvonik Corporation, Parsippany, N.J., USA) at magnifications of 2000×and 5000×, respectively. The silica particles are relatively small insize (e.g., much less than 1 μm) and form cloud-like agglomerates.

Image analysis is used to determine median aspect ratio, medianelongation, and median circularity of the foregoing materials. A MalvernMorphologi®G3 (www.malvern.com, Malvern, Worcestershire, UK) analyzer isused to determine these shape factors according to the Morphologi®G3instructions. A distribution of aspect ratios of the solid particulateadditives and silica is illustrated in FIG. 8. A distribution ofelongations of the solid particulate additives and silica is illustratedin FIG. 9. A distribution of circularities of the solid particulateadditives and silica is illustrated in FIG. 10.

Table 1, below, presents such shape factor data in terms of medianaspect ratio, median elongation, and median circularity.

TABLE 1 Shape Factor Data Shape Factor Graphite Mica Talc Silica MedianAspect Ratio 0.693 0.629 0.656 0.742 Median Elongation 0.303 0.368 0.3410.251 Median Circularity 0.918 0.834 0.880 0.931

This data indicate that the solid particulate additives of the sorbentcomposition disclosed herein have more elongate or rectangular shapethan they do a circular shape as compared to a more amorphous particlesuch as silica. The median aspect ratio of the graphite, mica, and talcare lower than that of the silica being further away from 1, indicatingthey are of a more rectangular than square in shape. The medianelongation numbers also show that the graphite, mica, and talc particlesare more elongate than silica. The median circularity of the graphite,mica, and talc particles also are significantly more divergent from acircularity of one, which indicates that these compositions are far froma perfectly circular particle type.

Example 3

Example 3 illustrates improved air flow capabilities of the sorbentcompositions of the present disclosure, and presents BFE, SE and AE dataof various samples. Table 1 below presents BFE and SE data, measured onan FT4 Rheometer using the SVFR method as described above, and AE datameasured on an FT4 Rheometer using the aeration test as described above.It is noted that an AE of 10 mJ would indicate complete fluidization ofthe sample in the instrument. Table 2 indicates that the use of thesolid particulate additives such as graphite, mica, and talc have asignificant effect on the BFE, SE and AE of a sorbent composition thatincludes PAC, even when added in relatively small quantities.

TABLE 2 Basic Flow Energy (BFE), Specific Energy (SE) and AerationEnergy (AE) BFE SE AE Sample Characteristic (size, additive) (mJ) (mJ/g)(mJ) 4 30 μm, no additive 329 7.18 17.0 2 15 μm, no additive 357 7.8837.5 3 15 μm, 1.0% graphite (bulk mix) 243 6.76 26.9 5 15 μm, 0.5%graphite (lab mix) 261 6.76 27.1 6 15 μm, 1.0% graphite (lab mix) 2537.02 21.6 7 15 μm, 0.5% mica (lab mix) 220 6.58 23.0 8 15 μm, 0.5% talc(lab mix) 219 6.70 24.4

The example compositions (Samples 3 and 5-8) with smaller medianparticle size (e.g., 15 μm or less) all show decreased BFE over thecomparative sorbent composition with reduced flow properties (Sample 2),and even lower BFE than the comparative composition of 30 μm medianparticle size (Sample 4). Similarly, the example compositions withsmaller median particle size (Samples 3, 5-8) show lower SE than boththe comparative Samples 2 and 4. The AE of each of the examplecompositions (Samples 3, 5-8) was significantly lower than that of thecomparative small median particle size with reduced flow properties,approaching the AE of the flowing larger median particle sizecomposition (Sample 4).

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present invention.

What is claimed is:
 1. A sorbent composition comprising a particulateblend of: at least about 50 wt. % powdered activated carbon sorbenthaving a median particle size (D50) of not greater than about 20 μm, asum of micropore volume plus mesopore volume of at least about 0.25 cc/gpore, and a ratio of micropore volume to mesopore volume of at leastabout 0.7; and at least about 0.1 wt. % and not greater than about 12wt. % of a solid particulate additive selected from the group consistingof phyllosilicate minerals, graphites, and mixtures thereof.
 2. Thesorbent composition recited in claim 1, wherein the solid particulateadditive is selected from the group consisting of mica group minerals,chlorite, serpentine, talc group minerals, clay minerals, and mixturesthereof.
 3. The sorbent composition recited in claim 1, wherein thesolid particulate additive comprises graphite.
 4. The sorbentcomposition recited in claim 3, wherein the graphite comprisescrystalline flake graphite, amorphous graphite, pyrolytic graphite, orcombinations thereof.
 5. The sorbent composition recited in claim 4,wherein the graphite comprises natural crystalline flake graphite. 6.The sorbent composition recited in claim 3, wherein the graphite has amedian particle size of at least about 3 μm and not greater than about 5μm.
 7. The sorbent composition recited in claim 2, wherein the solidparticulate additive comprises chlorite.
 8. The sorbent compositionrecited in claim 2, wherein the solid particulate additive comprisesserpentine.
 9. The sorbent composition recited in claim 2, wherein thesolid particulate additive comprises a clay mineral.
 10. The sorbentcomposition recited in claim 9, wherein the clay mineral is a calcinedclay mineral.
 11. The sorbent composition recited in claim 2, whereinthe solid particulate additive comprises an aluminosilicate.
 12. Thesorbent composition recited in claim 11, wherein the aluminosilicate isa calcined aluminosilicate.
 13. The sorbent composition recited in claim1, wherein the solid particulate additive has at least one particleproperty selected from: (i) a median aspect ratio of not greater than0.7; (ii) a median circularity of not greater than 0.92; and (iii) amedian elongation factor of at least 0.3.
 14. The sorbent compositionrecited in claim 1, further comprising an acid gas additive.
 15. Thesorbent composition recited in claim 14, wherein the acid gas additivecomprises a metal cation selected from Group 13 and Group 14 metals. 16.The sorbent composition recited in claim 15, wherein the metal cation isselected from the group consisting of aluminum, tin and combinationsthereof.
 17. The sorbent composition recited in claim 15, wherein theacid gas additive comprises an inorganic salt.
 18. The sorbentcomposition recited in claim 17, wherein the acid gas additive isselected from the group consisting of aluminum hydroxide, tin hydroxide,tin oxide and combinations thereof.
 19. The sorbent composition recitedin claim 14, wherein the acid gas additive comprises a calcium compoundselected from the group consisting of quicklime, hydrated lime, andcombinations thereof.
 20. The sorbent composition recited in claim 1,wherein the median average particle size (D50) of the powdered activatedcarbon sorbent is at least about 2 μm and is not greater than about 12μm.
 21. A sorbent composition comprising a particulate blend of: atleast about 50 wt. % powdered activated carbon sorbent having a medianparticle size (D50) of not greater than about 20 μm; and at least about0.1 wt. % and not greater than about 12 wt. % of a solid particulategraphite additive having a median aspect ratio of not greater than 0.7.22. The sorbent composition recited in claim 21, wherein the solidparticulate graphite additive is selected from the group consisting ofcrystalline flake graphite, amorphous graphite, pyrolytic graphite, andcombinations thereof.
 23. The sorbent composition recited in claim 21,further comprising an acid gas additive comprising an inorganic salthaving a metal cation selected from Group 13 and Group 14 metals. 24.The sorbent composition recited in claim 23, wherein the acid gasadditive is selected from the group consisting of aluminum hydroxide,tin hydroxide, tin oxide, and combinations thereof.
 25. The sorbentcomposition recited in claim 21, further comprising an acid gas additivecomprising a calcium compound selected from the group consisting ofquicklime, hydrated lime, and combinations thereof.
 26. The sorbentcomposition recited in claim 21, wherein the powdered activated carbonsorbent has a pore volume such that a sum of micropore volume plusmesopore volume is at least about 0.25 cc/g.
 27. The sorbent compositionrecited in claim 26, wherein the pore volume is such that ratio ofmicropore volume to mesopore volume is at least about 0.7.
 28. Thesorbent composition recited in claim 21, wherein the median averageparticle size (D50) of the powdered activated carbon sorbent is at leastabout 2 μm and is not greater than about 12 μm.
 29. The sorbentcomposition recited in claim 21, wherein the solid particulate graphiteadditive has a median particle size of at least about 3 μm and notgreater than about 5 μm.
 30. The sorbent composition recited in claim21, wherein the solid particulate graphite additive has at least oneparticle property selected from: (i) a median circularity of not greaterthan 0.92; and (ii) a median elongation factor of at least 0.3.